Heated transfer line for capillary tubing

ABSTRACT

A heated transfer line for heating a small glass capillary tube is disclosed. The transfer line is small and removable yet highly efficient, capable of uniformly heating a glass capillary tube over a 5-inch length to more than 400 degrees with less than 30 watts of power. The power is applied to an electrically conductive heater tube, which encircles the glass capillary, via first and second current conductors attached to respective ends of the heater tube. The first and second current conductors are arranged in proximity to the heater tube and back along the heater tube to a common point without touching each other. Electrical insulation is disposed between the heater tube and the first and second current conductors to electrically isolate the heater tube from the current conductors. A cover of thermal insulation is disposed over the heater tube and the current conductors and is used to thermally isolate the heater tube. An outer tube is disposed around the thermal insulation as a cover. A mounting ferrule is connected to the outer tube at the common point for mechanically mounting the transfer line and for providing an exit for the first and second current conductors.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 581,923, filed 2-21-84, nowU.S. Pat. No. 4,650,964, granted Mar. 14, 1987.

BACKGROUND OF THE INVENTION

The general evolution of gas chromatographs has led to the use of verysmall glass capillary gas transfer lines and columns. A typical glasscapillary tube has an inner diameter of 0.013 inch and 0.018 inch outerdiameter. Vaporized chemical samples are carried, usually in a carriergas, among the various devices in the gas chromatograph and betweenthese devices and other devices located in adjacent detectors or otherinstruments external to the gas chromatograph through heated transferlines.

The function of the transfer line heater is to maintain the vaporizedchemical sample within defined temperature bounds as it is carried amougthe various components of the analytical instruments. A particularproblem arises when the vaporized chemical samples are allowed to coolin the transfer lines, as this cooling can lead to precipitation of thesample out of the gas and onto the glass tube walls. Once this occurs,the chemical analysis is vitiated and there is the possibility that theglass tube can become plugged or that future chemical analysis can becontaminated. If heated above the temperature bounds, there is thepossibility that the chemical sample will chemically react or decomposealso vitiating the analysis. Therefore, it is important in gaschromatography that the glass capillary transfer lines be controllablyheated.

Transfer lines are also used as gas conduits between analyticalinstruments, for example, between a gas chromatograph and a peripheraldetector such as a spectrophotometer. For this use, the heated transferline should be temperature controlled and powered in one instrument andprotrude into the second. Since the small glass capillary tubes aredifficult to clean in an instrument and are subject to breakage, theapparatus which heats the capillary tube must also permit itsreplacement.

In general, heated transfer lines are designed to operate in atemperature range of 150-350 degrees C., concurrent with that of presentgas chromatographs. Since many chemical compounds have chemicalreactivity or decomposition temperatures near their boiling points, itis highly desirable that the temperature range, or tolerance of thetransfer lines be small, e.g. with 10 degrees C.

Several instrument oriented constraints on transfer line design have,heretofore, limited the temperature profile integrity of transfer lines.As a practical matter, it is desirable that a transfer line bridging twoseparate instruments be securely mounted, controlled and powered fromone instrument while simply extending in to the second. The degree ofmounting in the second instrument, if at all, may be determined by suchinfluences as the need to isolate the instruments from vibration, or theneed for instrument modularity. The external environment surrounding thetransfer line as it extends from within one instrument to within anothercan vary significantly. For example, a transfer line traversing from theinside of a heated gas chromatograph oven, through the oven wallinsulation and out to the ambient air gap between instrumentsexperiences a variation in temperatures ranging from perhaps 20 to 350degrees C.

Prior art transfer lines have been limited to the rather simple approachof inserting the gas capillary tube in comparatively large body of ahighly thermal conductive metal housing which is heated by an enclosedheater, such as a cartridge or band heater. This combination is furthersurrounded by a thermal insulator of some type, which tends to mitigatechanges in the surrounding environment. The larger the metal body andinsulation, the more uniform the temperature. Several disadvantages areevident in this approach. First, since the mass of the metal housing isprimarily designed to house the heater rather than the small glass tube,such heated transfer devices are particularly power inefficient andsubject to substantial heat loss in the surrounding instrument.Analytical detectors such as spectrophotometers are particularlysensitive to temperature gradients due to the precise alignment of theiroptics and can not afford the heat dissipation from a relatively largetransfer line operating at 350-400 degrees C., as is required in gaschromatography. Typically, these prior art transfer lines have diametersof 5 cm or more and due to their size, dissipate as much as severalhundred watts of power into their host instruments. These largediameters also inhibit coupling of the transfer line to instrumentdevices. It is not uncommon for cold junctions to develop at thecoupling which are 50 degrees C. or more below the desired operatingpoint.

Alternatively, prior art transfer lines have consisted of a weave ofhighly electrically resistant wire, such as an alloy of 80% nickel and20% chromium, and fibrous glass or ceramic insulation wound about theglass capillary tube into which the glass capillary may be inserted.Electrical current supplied to the metal wire heats the capillary.Although usually smaller in diameter than the previously describedtransfer lines, the woven transfer lines lack temperature uniformitywhen positioned in a temperature environment which changes along thelength of the transfer line. This is because there is insufficientthermal conductivity in the axial direction of the capillary tube.

Other prior art transfer lines have consisted of a tubular structureheated at one end, the capillary tube passing through the center of thetube. The heating element is housed in metal attached to the tube andheat is transferred by conduction down the length of the tube. Althoughthis approach provides for a smaller size on the non-heated end, itobviously suffers from non-uniform heating of the capillary tube. Tomaintain a minimum temperature at the cold end the heater must bebrought to a higher temperature than required for the chemical analysis.

SUMMARY OF THE INVENTION

In summary, the invention embraces an apparatus which permits gases tobe transferred from one device to another through a glass capillary tubewith minimal changes of temperature. The apparatus consists of a heatedtransfer line and an optional oven feedthrough.

The heated transfer line works by transferring the heat, which isgenerated when an electrical current is passed through a highlyresistive metal tube, to a glass capillary tube, which is inserted intothe metal tube.

The heated transfer line is small in size and low in power dissipationwhen compared to the prior art. The heater consists of severalconcentric tubes, each with a special purpose. The heater tube throughwhich current passes is small in diameter and only slightly larger thanthe glass capillary tube and is designed to efficiently transfer theheat directly to the capillary tube. The heater tube is electricallyinsulated, except for the ends where power is supplied through twoelectrical conduits or wires. These conduits form the second layer ofthe transfer line and also generate heat as a current passes throughthem. The conduit tubes are specially shaped in order to generate moreheat at the ends, without raising temperatures significantly in themiddle of the tube. Their length together is approximately equal thelength of the heater tube and permits power to be supplied at a centrallocation near the middle of the tube. At this juncture, a thermocoupleis mounted which is used to monitor the transfer line temperature. Theconduit tubes are covered with a thermal insulator and the entireassembly is enclosed within an outer tube.

The oven feedthrough consists of a short piece of copper pipe mounted toa thermal insulator with a hole in the center through which a heatedtransfer line, similar to the one described above, may pass. Theinsulator serves to insulate the copper pipe and heated transfer linefrom the oven. The inner diameter of the copper pipe is several timeslarger than the transfer line which because of its thermal conductivityattempts to keep a constant temperature over its length therebyproviding a more gradual change in temperature between the oven and theambient air for the heated transfer line.

The invention is superior to the prior art in several ways. Theinvention provides considerably more uniform temperature throughout thetransfer line than the prior art. The invention dissipates low power,typically 8 watts compared to several hundred watts of the prior art.The invention is small in size, the transfer line heater being 0.25inches in diameter rather than the 2 to 5 inches or greater typically ofthe prior art.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1C and 1D illustrate sectional, side and end viewsrespectively, of a heated transfer line for capillary tubing accordingto a first embodiment of the present invention.

FIG. 1B shows a portion of the heated transfer line shown in FIGS. 1A,1C and 1D.

FIGS. 2A, and 2A' illustrate exploded portions of a heated transfer linefor capillary tubing according to a second embodiment of the presentinvention.

FIGS. 2B, 2B' and 2C illustrate sectional and end views, respectively,of the heated transfer line shown in FIGS. 2A and 2A'.

FIGS. 3A and 3B illustrate the variations in thickness of the conduittube walls used to selectively heat the transfer line of FIGS. 2A-2D.

FIGS. 4A, 4B and 4C illustrate left, front, and right side views,respectively, of a capture ring for use with the heated transfer line ofFIGS. 2A-2D.

FIGS. 5A, 5B and 5C illustrate top, side and perspective views,respectively, of the solder lug used to connect an electric conductor tothe conduit tubes.

FIG. 6 illustrates an assembled view of a portion of the heated transferline of FIGS. 2A-2D.

FIG. 7 is an illustration in cross section of an oven feedthrough.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The heater tube is conveniently selected from hypodermic needle syringestock, usually hardened stainless steel. The small size of the glasscapillary diameter allows the use of small diameter syringe stock withthin walls, both combining to provide smallness of size and anelectrical resistance favorable towards its use as a heating element.The small size of the heater tube and its concentric location about theglass capillary results in a small uniform transfer line heaterconfiguration for a glass capillary which is removable. Likewise, withthis configuration power consumption is greatly reduced due to the smallexposed transfer line surface area required at any given temperature.

In order to supply a current to the heater tube, the heater tube isfirst electrically insulated over its external surface, then electricalwires, or other electrical conduits, are connected to the uninsulatedends of the heater tube and made to extend back over the length of theinsulated heater tube to some centrally located mutual point along itslength from which general electrical connection of the heated transfertube unit can be made externally. A temperature sensor is also coupledto the heater tube, and the leads of the sensor are also brought out atthe centrally located mutual point.

A first embodiment of the invention is shown in FIGS. 1A, 1B, 1C and 1D.A heater tube 100 made of fully hardened stainless steel or a highresistance nickel/chromium alloy tubing having a 0.031 inch outerdiameter and a 0.020 inch inner diameter is inserted into an aluminatube 110 having an outer diameter of 0.062 inch and inner diameter of0.032 inch as shown in FIG. 1A. The heater tube 100 extends out beyondthe alumina tube 110 0.09 inch on each end. Copper wires 120 and 123 of28 gauge size are wrapped about and silver soldered to the exposed ends125 and 127 of the heater tube 100 and each wire 120 and 123 is broughtto an intermediately located ferrule position 130 as shown in FIG. 1B. Athermocouple 140 is cemented with a high temperature ceramic cement onthe outer surface of the alumina tube 110 at the ferrule position 130.Lengths of a center drilled ceramic fiber tube insulation 150 and 155having an outer diameter of 0.210 inch and inner diameter of 0.062 inchare slid over and abutted to the alimina tube 110 and over the 28 gaugecopper wires 120 and 123. Yet another tube 160, a stainless steel sheathhaving outer diameter 0.250 inch and inner diameter 0.210 inch, as shownin FIG. 1A is slid over the assembly of FIG. 1B. The thermocouple 140and copper wires 120 and 123 are pulled out through the fiber tubeinsulation 150 and 155 and through holes 170 drilled raidally in thetube 160 at 120 degrees radial spacing. A stainless steel ferrule 180shown in FIG. 1A is slid over the tube 160 and positioned with the wires120 and 123 and the leads 141 and 142 of thermocouple 140 entering 120degrees spaced grooves 185 in the ferrule 180. The leads 120, 123, 141,and 142 are ceramic cemented in place and exit through 0.032 inch innerdiameter, 0.062 inch outer diameter alumina tube 190 via holes 192. Theferrule 180 is silver soldered to the tube 160. Alumina end caps 200 and205 are cemented with a high temperature alumina cement to each end 207and 209 of the tube 160. Onto end cap 205 is cemented a stainless steelfitting 210 having a thread. The fitting 210 and end caps 200 and 205each has a central hole passing therethrough so that a glass capillarytube (not shown) can enter through either end 212 or 214 of the thusformed heated transfer line 215 shown fully assembled in FIG. 1C andpass out the other end. Holes 220 located radially about the flange 230of the ferrule 180 as shown in FIG. 1D allowed the mounting of theheated transfer line 215 to a flat surface.

In operation the transfer line 215 of FIG. 1C is powered by passingcurrent through the copper wires 120 and 123 which thus allows currentto flow uniformly through the entire length of heater tube 100. Thepower dissipated in the heater tube 100 causes its rapid heating. Thesurrounding ceramic fiber insulation 150 and 155 restricts heat losswhile the ceramic tube 110 and ceramic end caps 200 and 205 isolate theheater tube 100 electrically from the tube 160 exterior and the copperwires 120 and 123.

The above described heated transfer line has a temperature capability inexcess of 400 degrees C. However, the metal ferrule 180 serves as anundesired heat sink creating a localized lower temperature in the heatertube 100 at the ferrule location 130. In addition, the construction islabor intensive.

A second and preferred embodiment shown in FIGS. 2A, 2A', 2B, 2B', and2C is now disclosed to overcome these difficulties while maintain boththe function and size of the first embodiment. A fully hardenedstainless steel or high resistance nickel/chromium alloy tube 250 havinga 0.031 inch outer diameter, 0.023 inch inner diameter and 5.676 inchlength is wrapped with a 0.062 inch wide by 0.001 inch thick polyimideinsulation tape 260 about the heater tube 250 periphery and along itslength to within about 0.25 inch of the end of tube 250 as shown in FIG.2A. A 0.001 to 0.0015 inch thick silicon based adhesive on one side ofthe insulation tape 260 allows adhesiion between the tube 250 and tape260. The adhesive is only needed during assembly, and actuallyevaporates during use.

A thermocouple 270 having 0.010 inch diameter leads 271 and 272 ispositioned on the tape 260 on tube 250 approximately 0.5 inch from oneend of tube 250. The thermocouple 270 and both of its leads 271 and 272are held in place over opposing sides of the insulated heater tube bysliding the capture ring 280 over the thermocouple 270, and covering theopen end of the capture ring 280 with the polyimide (e.g., Vespelavailable from DuPont Corporation) washer 281 thereby trapping thethermocouple against the insulated heater tube and between the washer281 and the capture ring 280. Top, left side and right side views ofcapture ring 280 are shown in FIGS. 4A, 4B, and 4C respectively andthermocouple leads 271 and 272 will exit through slots 285 in capturering 280. A typical capture ring 280 will have the following dimensions:

d1=0.094 inches in diameter

d2=0.018 inches

d3=0.015 inches

d4=0.059 inches in diameter (centerbore)

d5=0.035 inches

d6=0.038 inches

d7=0.064 inches

d8=0.034 inches

d9=0.034 inches.

The function of capture ring 280, in addition to positioning and holdingthermocouple 270 directly against the tape 260, is to position the leads271 and 272 of the thermocouple 270 and to electrically isolate themfrom each other, and to thermally insulate the heater tube 250 from itssurroundings.

Electric current is supplied to the heated transfer line through leadwires 290 and 295. Typically, wires 290 and 295 are 22 gage singlestrand silver plated copper wires insulated with Teflon. Electrical leadwires 290 and 295 are connected to the two respective solder lugs 310and 315 by crimping and silver soldering the respective joints. Becausethe power required to heat the transfer line is quite small, a verysmall increase in power or the addition of a relatively small heat losscan drastically affect the local temperature of the transfer line.Therefore, the goal of the connector design is to provide a goodelectrical connection which is thermally invisible to the heater andconduit tubes. Since the connector would normally conduct heat away fromthe heater tube, this may be accomplished by passing a current throughthe connector which must heat the connector such that its temperatureand thermal conductivity combine to provide an apparent thermalresistance which matches that of the insulation which surrounds theheater and conduit tube. FIGS. 5, 5B and 5C illustrate solder lugs 310or 315 whose dimensions meet the above conditions by use of a preciselydimensioned ribbon 510 between the connecting points 520 and 530. Atypical solder lug 310 or 315 will have the following dimensions:

f1=0.090 inches in diameter

f2=0.052 inches in diameter

f3=0.070 inches

f4=0.022 inches

f5=0.010 inches in radius

f6=0.125 inches

f7=0.200 inches

f8=0.030 inches

f9=0.0050 inches.

The two solder lugs 310 and 315 are then attached to the ends of theirrespective conduit tubes 300 and 305. Silver soldering is chosen torestrict high temperature corrosion at the electrical crimp. Conduittubes 300 305 are typically stainless steel tubes having an outerdiameter of 0.072 inch and inner diameter 0.040 inch. Once the solderlugs 310 and 315 are attached, conduit tubes 300 and 305 are slid overheater tube 250 oriented with the solder lugs at 120 degrees as shown inFIG. 6. With this orientation, the unattached ends 330 and 335 ofconduit tube 300 and 305 are silver soldered to the exposed ends 337 and339 of heater tube 250. The insulation tape 260 serves to electricallyinsulate heater tube 250 from conduit tubes 300 and 305 such thatcurrent will be allowed to travel down the conduit tubes 300 and to thesilver solder joint 330, then through the heater tube 250 to the solderjoint 335 and back out through tube 305. FIG. 6 illustrates thepartially assembled transfer line where the solder lugs 310 and 315 andthermocouple mounting hardware meet.

Short and long outer tubes 340 and 345, respectively are inserted intoferrules 350 and 355, respectively as shown in FIGS. 2A and 2A' andstaked into place. Staking occurs by flaring out the exposed end of eachtube into the respective ferrules to prevent removal. Ferrules 350 adn355 are constructed from polyimide for purposes of temperatureresistance and to further restrict the flow of heat out of the assembledtransfer line heater 240 shown in FIGS. 2B and 2B'. Teflon flex tubing360 is inserted over each lead 271 and 272 of thermocouple 270 andcrimped in place with crimp 370. During assembly, crimp 370 fits into anenclosed cavity 380 in ferrule 350 such that the flex tubing 360 is noteasily pulled out of the assembled transfer line heater 240. Eachferrule 350 and 355 with tube 340 and 345 is slid over their respectiveends 335 and 330 of the previous assembly, as shown in FIG. 2A. Eachwire 290 and 295 and the leads 271 and 272 of thermocouple 270 arepositioned respectively in grooves 390, 395, and 397 in upper ferrule350. Ceramic fiber insulating tubes 400 are slid over the ends 335 and330 of the above assembly in the annulus formed between the electricalconduits 305 and 300 and the outer sheathes 340 and 345. The insulationtubes 400 are pushed down against capture ring 280 and cover 281 and fitsnugly adjacent to each other. The outer most insulating tube 400 istrimmed to allow the total proper length of insulation. Ferrules 350 and355 are then riveted together with eyelets 410. The center holes ofeyelets 410 serve as holes for mounting of transfer line heater 240.Polyimide insulators 420 and 425 are slid and positioned over the ends335 and 330 of conduits 300 and 305. Insulators 420 and 425 serve toelectrically insulate the conduits 300 and 305 from the outer tubes 345and 340. End caps 440 and fitting 450 are made of stainless steel andare dipped in liquid nitrogen then quickly tapped into their respectiveends 460 and 465 of the outer tubes 340 and 345. The expansion fit formsan interfering fit when the end cap 440 and fitting 450 are warmed toroom temperature in the outer tubes 340 and 345.

A uniform tube generates uniform temperature rise along its length whenan electric current is passed through the tube. A temperature profile ofan operating uniform tube reveals however an uneven temperature profiledue to uneven heat transfer to the environment. This problem isparticularly severe toward the ends of the tube. As a major improvementof the present heated transfer line, its uniform temperature isaccomplished by varying the wall thickness of the conduit tubes alongthe length of the transfer line heater. At any given position along saidlength the total power per segment is equal to the power losses of theheater tube and the power losses of the surrounding conduit tube. Sincethe current I is identical for each segment the power input per segment,q, is:

    q=I.sup.2 (Rht+Rct)                                        <1>

Where Rht and Rct are the electrical resistances of the heater andconduit tubes respectively. The power loss from the heater tube to theenvironment varies along the length of the tube, for example, the endlosses are greater than losses at the center of the transfer line.Therefore in order to keep a uniform temperature, it is desirable to addadditional power to those segments with greater losses. Referring toequation 1, this can be accomplished by locally increasing theelectrical resistance in those segments with greater losses. Theresistance in a given segment may be increased by adjusting the wallthickness of any of the heating tubes. Since in the preferred embodimentthe heater tube has assumed a minimal practical size for the overallresistance of the transfer line, a wall thickness adjustment is mosteasily made by altering the outer diameter of the conduit tubes.

This effect can be expressed mathematically in simple form by noting theelectrical circuit analogy to the field of thermodynamics. The heatertube temperature To is related to the ambient temperature Ta, the powerinput per segment q and the thermal resistance between To and Ta of Rthby the equation:

    To=qRth+Ta

    To=I.sup.2 (Rht=Rct)Rth+Ta                                 <2>

At the transfer line midpoint the thermal resistance Rth consistsprimarily of a sum of series resistances for a thermal dissipation pathwhich extends radially out from the heater tube. Moving toward the ends,a parallel resistance enters into Rth due to axial losses (assuming theends are cooler). This effectively lowers Rth, consequently lowering Tofor a constant electric current I. As Rth varies, the electricalresistance of the conduit tube, Rct, can be varied to compensate byaltering preferably the outer radius of the conduit tube at the affectedsection. For a tube the electrical resistance can be calculated as:##EQU1## where p is the resistivity of the tube material at theoperating temperature To, L is the evaluated tube length, pi is 3.14 . .. , and do and di are the outer and inner tube diameters, respectively.

Thermal resistance can be calculated in simple configuration but may bemore comples when all parameters are not known. The temperaturedistribution along a transfer line can easily be measured, however, byinserting a 0.010 inch diameter thermocouple probe into the end of aglass capillary and traversing the assembly down the length of theheater tube when operating.

Assuming the following values:

Rht: do=0.031" di=0.023" p=3.8386e-5 ohm-inch

Rct: do=0.063" di=0.038" p=3.8386e-5 ohm-inch

Ta=25 degrees C. and To=300 degrees C.

and, due to practical grinding restrictions, a minimum outer diameterdo=0.050", then applying the above formulas 2 and 3 it may be shown thata temperature rise of 45 degrees C. is possible. In the preferredembodiment an even larger tube was used to obtain an even higher temprise.

This analysis works well as a first order approximation, although anexact solution is not achieved since an adjusted segment is affected byadjoining unadjusted segments and due to changes in Rth which aretemperature dependent.

FIG. 3A, which is a picture of conduit tube 300 having an overall lengthof 5.236 inches, shows the various reductions in diameter for aparticular application of the preferred embodiment. At point 803, 0.450inches of length of the tube diameter (0.072 inches) is unchanged. Thetube diameter is ground down 0.009" in sections 802 (0.750 inches long)and 804 (0.450 inches long) and further ground to 0.053" in section 800,and still further ground to an outer diameter of 0.050" for a length of0.020 inches at point 801. FIG. 3B shows the similar grinding on conduittube 305 having an overall length of 0.540 inches. The tube diameter(0.072 inches) is not ground at 810 for a length of 0.250 inches, it isground down 0.009" at 811 (for a length of 0.140 inches) or 812 (for alength of 0.061 inches) and still further ground to 0.50" at section 813(for a length of 0.089 inches).

The second embodiment of the transfer line heater 240 as shown in FIGS.2B, 2B' and 2C achieves uniform heating to 400 degrees C. with less than10 watts of dissipated power with a variation of less than 10 degrees C.

Apparent variations in the external temperature environment of thetransfer line when mounted in various gas chromatograph ovenconfigurations has also led to a need to improve the method ofconnecting the transfer line to the oven while minimizing gaschromatograph thermo-mechanical differences in transfer lineperformance. An additional feedthrough device for this purpose is shownin FIG. 7. The feedthrough consists primarily of a copper tube 903 whoseinternal diameter is substantially larger than the transfer lineheater's outer diameter. The copper tube mounted in the gaschromatograph oven wall insulation 901 strives to be at constanttemperature along its length. The copper tube is purposefully oversizedto allow sufficient thermal grounding of the tube to the ambient toreduce the variations in temperature of the tube along its length, andalso, to allow for variations in oven wall thickness among the variousgas chromatograph instruments.

To reduce the thermal sink effect of the copper tube on the oven itself,the copper tube is not allowed to protrude through to the oven interior,but instead extends into a refractory ceramic fiber block 902 containinga centrally located hole to allow the axial passage of the heatedtransfer line 904 into the oven. The block also serves to isolate theheated transfer line from exposure to the high temperature oven and toprevent convection flow out of the oven and over the transfer line. Useof the oven feedthrough permits a heated transfer line with less than aplus or minus 10 degress C. variation when used with a gaschromatograph.

I claim:
 1. An apparatus for heating a length of tubing to a uniformtemperature with an electric current, the apparatus comprising:a heatertube which is electrically conductive and has first and second ends;first insulating means substantially covering the length of the heatertube for electrically insulating the heater tube; first and secondelectrically conductive conduit tubes having a combined lengthsubstantially equal to the length of the heater tube, said first andsecond conduit tubes covering the first insulating means and each havingan outer end electrically connected, respectively to the first andsecond heater tube ends, the opposite inner ends of the conduit tubescoming into closely spaced proximity at a common point along the heatertube; first and second conductor means electrically connected to thefirst and second conduit tube inner ends near said common point forsupplying electric current to said heater tube through said first andsecond conduit tube; second insulating means surrounding and thermallyinsulating the first and second conduit tubes; first and second outertubes substantially covering the second insulating means and havingfirst and second outer tube ends with the opposite inner ends of theouter tubes coming into closely spaced proximity at said common pointalong the heater tube; and mounting means connected to the outer tubeinner ends in a mounting plane which passes through said common pointalong the heater tube mechanically mounting the apparatus and providingan exit for the first and second conductor means.
 2. An apparatus as inclaim 1 wherein the heater tube is composed substantially of a highresistance nickel/chromium alloy.
 3. An apparatus as in claim 1 whereinthe first and second conduit tubes are composed substantially of astainless steel.
 4. An apparatus as in claim 1 wherein each of the firstand second conduit tubes are composed of a tube with at least onepredetermined variation in tube wall thickness.
 5. An apparatus as inclaim 1 wherein the thermal resistances of both conductor means areapproximately equal to the thermal resistance of the second insulatingmeans.
 6. An apparatus as in claim 1 further comprising a temperaturesensor thermally connected to the heater tube and having leads exitingthe appartus substantially in the mounting plane.
 7. An apparatus as inclaim 1 further comprising first and second tube fittings connected tothe first and second outer tube ends.